Gamba grass (Andropogon gayanus) was introduced to Australia as a pasture grass from Africa. It has become a significant environmental weed and is considered an ecosystem transformer, posing multiple threats to savanna ecosystems of northern Australia. Of particular importance is its capacity to increase fuel loads, resulting in intense fires that may subsequently lead to permanent transformation of the structure and diversity of the ecosystems it invades. Despite its listing as a key threatening process under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999, gamba grass has continued to spread through the Northern Territory, Queensland and Western Australia.
One of the major problems limiting the effective management of gamba grass once it’s established as an environmental weed is the lack of registered herbicides for use in natural systems and conservation areas. Glyphosate is the primary herbicide in use in northern Australia and the only herbicide registered for use on protected land. However, non-selective herbicides such as glyphosate have significant off-target impacts on native vegetation. Other herbicides have been trialled with varying success as soil and/or foliar applications. However, none have consistently proven effective at recommended rates of application or if they are effective, they also have negative impacts on non-target species.
There are numerous logistical issues with the widespread use of glyphosate in the north – application relies on large, heavy and expensive spray equipment as well as access to clean water which can be difficult to manage in remote areas. The herbicide must be applied during the hot and rainy growing season which often limits or prevents access. Additionally, glyphosate has no residual action and its effectiveness relies on follow-up treatments, and there are emerging resistance issues. Alternative herbicides are critical as insurance against future limitations on the use of glyphosate, to combat resistance, and to allow effective, timely control in the ecosystems of Cape York Peninsula and across northern Australia.
This project sought to determine whether there was a suitable alternative to glyphosate that selectively controls gamba grass with low off-target effects on native vegetation.
This project sought to identify alternative herbicide options for gamba grass by:
Various pots at 13 weeks after treatment. Photo: Queensland Department of Agriculture and Fisheries.
The herbicide rates and applications used in our field trials were designed to be consistent with the rates and methods used by local land managers who are largely controlling a low-density gamba infestation on foot, once per year. There are several potential reasons why the treatments used here in the field did not result in longer-term reductions in gamba cover. One application is clearly not sufficient to provide effective control by any of the herbicides we tested, and follow-up applications over multiple years are likely to be required. Higher rates of application are likely to be more successful in reducing cover; however, they are also likely to have more off-target effects.
The pot trials identified some herbicides which were not effective in suppressing gamba grass and also had low off-target effects at the rates applied. For these herbicides, higher rates could be trialled to determine if there is an optimum application rate which will selectively control gamba germination while minimising off-target effects. In contrast, the herbicides which did effectively suppress gamba germination also had high off-target effects on native species germination.
Although neither the field nor pot trials identified suitable herbicides that selectively controlled gamba grass with low off-target effects, there are several herbicides that warrant further testing at a range of application rates and in a range of environments. These include flupropanate (liquid and granular), clomazone, oxyfluorfen, imazapyr and indaziflam.
Further tests could also include a broader range of environments and co-occurring native species to identify when and where particular herbicides are most effective.
Currently, integrated management remains the best option for managing and eradicating gamba incursions and infestations. This includes various combinations of restricting seed movement, herbicide application and other management practices such as grazing, burning and slashing. Success of integrated management is often context-specific; however, increasing our understanding of how herbicides behave in combination with other management practices will be critical to controlling the spread of gamba grass in northern Australia.
Gamba grass can form dense stands up to 4 m tall, such as this one in the Northern Territory. Photo: NESP Northern Australia Hub.
This project was led by Dr Helen Murphy from CSIRO, with field monitoring by CSIRO’s Matt Bradford and Andrew Ford. Dr Murphy was assisted by Queensland Government staff, particularly form the Hann Tableland National Park, as well as researchers from Charles Darwin University and The University of Western Australia.
This project was completed in June 2021.
Helen Murphy, CSIRO